The invention relates to an apparatus and to a method for recording medically prepared teeth, in particular tooth models. Tooth models are characterized by steep walls and filigree structures and can include narrow and deep holes. Furthermore, when measuring tooth models it is necessary to record edges and steep surfaces with precision.
The measurement of a tooth model according to the triangulation procedure with a certain parallax angle, which is as small as possible, between the projection path of the rays and the observation path of the rays is known from EP-A-0 455 855. During the measuring process the model undergoes helical scanning by a triangulation sensor arranged in a stationary position and set to reveal sufficient detail. Also known in the art is the practice to mount this optical measuring instrument on one of the tool support heads allowing the sensor to be brought into the best possible measuring position by way of defined movements. The tool spindle movements are known and can be superimposed over the actual measured values of the sensor as a correction. A focus detection sensor is proposed as an example for a sensor to be used. The type and design of the measuring method remains unspecified.
Known from the xe2x80x9cDental CAD/CAM GM-Ixe2x80x9d brochure, authored by the GC Corporation of Tokyo, is the measuring of a tooth restoration in which a laser is able to measure said tooth restoration using a 5-axis measurement. The measuring unit is a separate component of a machine group for producing tooth restorations which further includes a separate grinding machine and a computer for process control.
It is the objective of the invention to measure medical objects, in particular to measure models of prepared teeth, in a simple manner.
An apparatus for recording medical objects, in particular for recording models of prepared teeth, is devised in accordance with the invention as comprising a position-sensitive sensor that is movable in the projection direction comprising a projection unit and a receiving unit; and between the projection path of the rays of the projection unit and the observation path of the rays of the receiving unit there is a certain triangulation angle, while the projection path of the rays is aligned parallel to the moving direction of the sensor. Since the triangulation angle is less than 10xc2x0, and preferably less than 5xc2x0, it is possible to measure with sufficient accuracy inside the tooth cavities allowing for the exact recording of edges and steep surfaces. Due to the fact that the receiving unit has a strip comprising several pixels, which are adjacent to each other and which can be evaluated individually, acting in conjunction with the signal processing, it is possible to take into account double reflections and divergence of the light spot at the edges when determining the ray""s center of gravity. Thus, the disadvantages of the otherwise widely used position-sensitive sensors (PSD=position-sensitive device) can be avoided. CCDs or photodiodes limiting the determination of the ray""s center of gravity to the calculation of only few pixels are advantageous. In order to be able to make adjustments for alignment errors of the observation path of the rays in relation to the strip on the receiving unit a medium allowing the divergence of the ray is arranged in a direction that is perpendicular to the strip. The sensor is movable in the projection direction to such an extent that it measures, with respect to the measuring accuracy, the same object point while moving in the projection direction.
If media for redirecting the path of the rays are arranged in the projection path of the rays and/or the observation path of the rays it is still possible to make enough room for the apparatus comprising the projection unit and the receiving unit despite the small triangulation angle.
Because the sensor is movable in the projection direction it is sufficient if the measuring range of the sensor covers at the most one tenth of the largest possible object dimensions to be measured in the projection direction.
It is advantageous if the projection unit comprises a laser diode with collimator optics and a selector of the laser diode with the radiation capacity being controlled by way of a pulse width modulated selector (PWM) whose pulse duty factor is calculated on the basis of the signal of the receiver. Pulse width modulation refers to the average laser intensity being adjusted by way of the time ratio between switched on laser and switched off laser.
The collimator optic comprises lenses that focus the laser beam on a certain point.
Another feature of the invention is an apparatus for recording medical objects with the sensor arranged inside a housing. Envisioned in the housing is a window for the projection path of the rays and/or the observation path of the rays with the window being heated to a higher temperature than the ambient temperature.
An apparatus of this type can be advantageously arranged inside a grinding chamber of a grinding machine on a spindle motor and in close thermal contact with the motor. Due to the close thermal contact with the spindle motor, which is the main source of heat during the grinding process, the window assumes for the most part the spindle motor""s temperature, which means that even in moist, warm conditions are prevalent inside the chamber of the grinding machine a fogging up of the window, and thereby a falsification of the measures values of the sensor, is avoided.
Using two receiving units will improve the measuring accuracy in particular with regard to the measuring of the edges. For this purpose it is advantageous to arrange the two receiving units symmetrically in relation to the projection path of the rays in order to compensate for the asymmetrical geometry of the triangulation procedure. With regard to the receiver signals it is, respectively, the more meaningful signal, e.g. in terms of its higher intensity or a clearer focus image, that is given more weight. Even though the compactness has to be sacrificed with the additional observation path of the rays, the enhanced measurement can take priority over compactness in special applications.
Another feature of the invention is a method for measuring medical objects, in particular for measuring tooth models, and the measuring is carried out according to the triangulation procedure with a certain triangulation angle between the projection path of the rays and the observation path of the rays. It is possible to compensate for the disadvantages resulting from the small triangulation angle because in a first step a sensor is moved in the projection direction by a drive apparatus in such a way that the object to be measured is brought inside an area +/xe2x88x92B which is part of a working distance a; and in a second step the sensor is moved along the object that is to be measured; and in a third step a measuring signal generated by the sensor is checked in order to determine if the signal is inside a range +/xe2x88x92B in relation to the working distance a; and in a forth step, if a measuring value is outside of the range B, the position of the sensor is modified to the extent that the sensor comes to be in the range +/xe2x88x92B, which means that preferably the working distance a is achieved; and in a fifth step a measuring value is generated, subsequently the object position is calculated based on the movement of the sensor and the measured value of the sensor. Although positioning in accordance to the Scheimflug rule is not possible for spatial reasons, which is why the working area is considerably limited, it is possible to measure with sufficient accuracy.
Preferably the radiation capacity of the projecting ray is controlled in such as way that the observing ray generates an approximately constant signal, in particular a laser diode is controlled to ensure that the amplitude of the signal on a photodiode strip is approximately constant.
To increase the accuracy of the measurement it is possible to measure the intensity and/or the half intensity width of the luminous spot generated by the projection unit and which hits the receiving unit. It is possible to flag the measured value if the intensity and/or the half intensity width are not inside the normal range.
In accordance with an advantageous further development of the invention the measured values that are afflicted with a systematic error are flagged as possibly faulty and are interpreted accordingly by the subsequent evaluation routines. It is thus avoided that erroneous measured data is incorporated with the final result.
In order to improve the measuring accuracy the stray light can be measured prior to every actual measurement and compensated for when the measured value is determined.
It is advantageous if, initially, the measured object is scanned only roughly and displayed on an output device. Thereafter, certain areas are selected and precise measurements are produced for these specified areas only. This saves considerable amounts of time because it is not necessary to record the entire measured object.
Furthermore, it can be advantageous if, utilizing the rough measurement, interactive steps are implemented and all the while the actual measuring process continues to run in the background.